High frequency pulse generating circuit

ABSTRACT

A circuit for generating high frequency output pulses comprising a magnetron, a pulse transformer having its secondary winding coupled to apply firing pulses across the input terminals of the magnetron, charge storage means connected in circuit with the primary winding of the pulse transformer, and switching means for discharging the charge storage means through said primary winding to generate a firing pulse from said secondary winding, said switching means comprising an FET switching device having output terminals connected in series with said primary winding, and a control terminal, and a control circuit adapted to deliver low voltage (e.g. less than 20 volts) pulses to said control terminal.

FIELD OF THE INVENTION

This invention relates to a circuit for generating high frequency outputpulses utilising a magnetron and has particular, though not exclusive,application in the generation of radar pulses.

BACKGROUND OF THE INVENTION

In order to energise a magnetron, so that it will oscillate and generatea high frequency output pulse, it is necessary to apply across its anodeand cathode a voltage pulse normally exceeding 1 kilovolt, which may bereferred to as a firing pulse, and the magnetron then oscillates for aperiod approximately equal to the duration of the firing pulse. Thepower of the firing pulse normally will exceed 1 kilowatt. Because ofthe relatively high voltage and power involved, there have been specialproblems associated with developing circuitry capable of switching afiring pulse onto a magnetron. The circuits employed have generally beenreferred to in the art as modulators. One of the desirablecharacteristics of a modulator is that is should be capable ofgenerating firing pulses of different lengths, so as to enablegeneration of magnetron output pulses of corresponding differentlengths.

One type of modulator which has been employed is referred to as the linetype modulator. This type utilises a delay line to define the firingpulse length, and in order to change this length more than one delayline is provided, these having different delay values, and relayswitches are employed to switch between one line and another. The energypassing through the circuit at the point of switching is relatively highso that the problems normally associated with mechanical switching atrelatively high energy levels occur. The output pulse from the delayline is applied to a pulse transformer which steps up its voltage forapplication across the magnetron. The output impedance of a line typemodulator is roughly equivalent to that of the pulse transformer, withthe result that only about half of the voltage which might nominally beexpected can actually be made available across the transformer primarywinding. Consequently, the turns ratio of the pulse transformer has tobe increased to achieve the desired output voltage in the firing pulse.The higher the turns ratio, the narrower the range of different pulselengths that may be satisfactorily transmitted through the pulsetransformer.

Another prior type of modulator is known as the hard valve typemodulator. This employs a high voltage vacuum valve which switches avoltage of several kilovolts directly across the magnetron. The hardvalve has the advantage that it can be switched from conduction tonon-conduction by means of a relatively low control voltage, and theoutput pulse length can be varied by varying the length of the inputcontrol pulses at this relatively low voltage level. However, the valveis large, requires typically between 10 and 20 watts of power forheating of its cathode, and requires a constant supply voltage ofseveral kilovolts to be provided for switching on to the magnetron.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved circuit forgenerating high frequency output pulses from a magnetron.

The invention provides a circuit for generating high frequency outputpulses comprising a magnetron, a pulse transformer having its secondarywinding coupled to apply firing pulses across the input terminals of themagnetron, charge storage means having substantially no inductanceassociated therewith and connected in circuit with the primary windingof the pulse transformer, and switching means for discharging the chargestorage means through said primary winding to generate a firing pulsefrom said secondary winding, said switching means comprising an FET(field effect transistor) switching device having output terminalsconnected in series with said primary winding, and a control terminal,and a control circuit adapted to deliver pulses to said control terminalwhereby to produce firing pulses of a length defined by that of pulsesfrom the control circuit. The pulses may be of less than one watt inpower, and preferably are less than one milliwatt. It will beappreciated from this that the FET should be a power FET (i.e. one ofthe types capable of handling at least 1 ampere), and a power MOSFET ispreferred.

In the preferred embodiment which will be described, the FET switchingdevice is an n-channel enhancement mode VMOS power FET. Further, thecontrol circuit is adapted to deliver pulses whose lengths arecontrollable, for example by means of a control signal.

The embodiment to be described has the advantages that the switching orcircuit modification required to modify the pulse length is effected ina low-power part of the circuitry,no switching contacts in thehigh-energy path are required, the output impedance of the FET isrelatively low thereby enabling a reduction of turns ratio on the pulsetransformer which in turn enables the transformer to handle a widerrange of pulse widths, and as compared with a hard valve modulator, itmay be as much as 20 times smaller, requires no heating power, and doesnot require a constant supply voltage in the kilovolt range. Ordinarylow-voltage solid state circuitry can be employed to generate thecontrol pulses for switching the FET.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood an embodimentthereof will now be described, by way of example, with reference to theaccompanying drawings.

In the accompanying drawings, which are diagrammatic:

FIG. 1 shows the components which are located in the display unit,

FIG. 2 shows the components which are located within the scanning unit,the components in FIG. 2 being linked to those in FIG. 1 by conductorsas indicated,

FIG. 3 is a timing diagram showing the various waveforms that occur invarious parts of the circuitry, these being identified by referenceletters in the Figure, and the same reference letters being used toindicate where the waveforms occur in the circuitry of FIGS. 1 and 2,and

FIG. 4 shows in more detail the circuitry which is used for storing theenergy required to fire the magnetron, and for firing the magnetron, asmore generally illustrated in FIG. 2.

GENERAL DESCRIPTION

Before referring to the structure and operation of the circuitry in anydetail, the general principles involved will first be briefly describedwith reference to FIGS. 1 and 2 so as to assist in an understanding ofthe detailed explanation that will follow.

The radar operates continually in repeated cycles, at 1000 or morecycles each second. At the beginning of each cycle a very brief (forexample, between 100 nanoseconds (ns) and 1 microsecond (μs)) pulse ofvery high frequency energy (for example at about 9450 MHz) is generatedby a magnetron 2 (see FIG. 2) and this pulse of energy is transmittedfrom the transmitting section 4a of a scanner generally indicated at 4.The transmitted pulse is reflected from any objects which lie in itspath and the reflected pulse or pulses, which may be referred to asechoes, are received at the receiving section 4b of the scanner 4.

A receiver, generally indicated at 6, converts the received very highfrequency echo pulse to a demodulated video signal which is applied tothe video input of a cathode ray tube (CRT) 8 which is shown in FIG. 1.

The beam of the cathode ray tube scans from its centre out to itsperiphery, the scan being commenced at the moment when the pulse istransmitted from the scanner. The video signal from the received echobrightens the scanning spot when it has travelled a distanceproportional to the distance of the object which caused the echo.Consequently, as the spot makes a single sweep radially away from thecentre of the tube face, bright points occur on the display at radialdistances proportional to the distances away from the scanner 4 of anyobjects which have caused echoes of the original transmitted pulse to bereturned to the receiving section of the scanner.

The scanner 4 is directional, and consequently any individualtransmitted pulse will only produce echoes from objects which lie alongthe bearing on which that pulse was transmitted. In order to obtain onthe face of the CRT 8 an indication of the features which lie in alldirections around the scanner 4, the scanner is continually rotated bymeans of a motor M1 (FIG. 2) and at the same time the scanning coils 10(FIG. 1) of the CRT 8 are rotated around the axis of the CRT by means ofa further motor M2, the mechanical link between the coils and motor M2being shown by a broken line 12. The rotation of the motors is kept insynchronism so that whenever a pulse is being transmitted in oneparticular direction the CRT spot is always travelling in acorresponding angular direction on the face of the CRT. As the scannerrotates, each successive pulse is transmitted in a slightly differentangular direction and the angular position of successive scans on theCRT screen is similarly incremented, due to the synchronism of the twomotors. The scanner and the CRT coils rotate on the order of once persecond (the motors being geared down as necessary to achieve this) andthe phosphor on the CRT screen persists in luminescence for at leastthis long, so that to the eye the large plurality of bright spotsappearing on each of the angularly incremented sequence of scans on thetube face form a complete picture of the radar-reflecting features whichsurround the scanner.

Normally, it is required for the CRT screen display to be able to showthe surrounding area on different scales. For example, when navigatingin restricted conditions, such as entering a harbour, it may bedesirable for the whole CRT screen area to show the surrounding area outto only half a mile radius, whereas in open sea it may be desirable forthe screen to show the area out to, for example, 16 miles radius. Inorder to achieve such different ranges, the speed at which the CRT spotscans may be changed. It will be evident that the faster the spot scansfrom the screen centre to the screen periphery, then the closer to thescanner will be any object whose image appears on the screen at itsperiphery i.e. the whole screen area will be occupied by images fromwithin a relatively shorter range around the scanner. Conversely, if thebeam spot is made to scan more slowly, images appearing around thescreen periphery will be from relatively more distant objects and thescreen will be displaying an image of an area extending for a greaterrange around the location of the scanner. A portion of the circuitry inFIG. 1 which is employed to vary the rate of scan on the CRT face isgenerally indicated at 13.

At short range, it is desirable for the pulses transmitted from themagnetron to be very short because otherwise there will be substantialloss of definition on the CRT display, since a long received echo wouldextend over a substantial radial distance on the very fast-movingscanning spot. At longer ranges, longer pulses are desirable to producea large enough bright spot with the spot travelling more slowly, and itis further advantageous to use longer pulses at longer ranges toincrease the mean energy receivable in the echoes, which tend to be farweaker from distant objects than they are from near objects. Thecircuitry of FIGS. 1 and 2 contains components which vary the width ofthe transmitted pulse such as to match the range, or spot scanningspeed, currently being used at the CRT.

To enable the user of the radar to estimate the distance of an objectrepresented by an image on the screen, it is commonly required for thescreen to continually display a plurality of rings concentric about itscentre, and equally spaced. Circuitry for producing these rings bybrightening the scanning spot at equal intervals as it travels away fromthe screen centre is generally indicated at 14 in FIG. 1.

A further facility normally required is a so-called "sea clutter"control. In rough weather at sea, the waves very close to the scannercause substantial echoes and this results in an area immediatelysurrounding the screen centre being continually bright, which isnormally undesirable. To avoid this, the gain of the receiver circuitryis substantially reduced during the initial portion of each sweep sothat the screen centre area is reduced in intensity. The portion of thecircuitry for achieving this is generally indicated at 15 in FIG. 1.

A detailed description of the circuitry and its operation now follows.

RADAR PULSE TRANSMISSION

A pulse repetition frequency generator 16 generates a square wave signalat a fixed frequency of 4000 Hz. The signal is shown at A in FIG. 3.This is applied to a divider 18 having outputs at which frequencies of4000 Hz, 2000 Hz, 1000 Hz and 500 Hz are delivered, see waveforms A, B,C and D in FIG. 3. Signal D at 500 Hz is applied to a monostable circuit19 which in response generates pulses E for application to an extra hightension circuit 20 which may be of known design and which via line 22applies a voltage of several kilovolts to the anode of CRT 8.

Any one of signals A, B and C can be selected from the outputs ofdivider 18 by means of a range switch section 24a. It will be assumedfor the purpose of the following explanation that it is signal C at 1000Hz which is being selected by range switch section 24a (this would beappropriate for the longest two ranges available, 8 miles and 16miles--see under "Range Changing" below). Throughout the circuitry thereare further range switch sections 24b, 24c and 24d and all thesesections are ganged together so that when the range switch is operatedsimultaneous changes are made at various points in the circuit so as toadapt it for operation at different ranges. They are shown set for the 8mile range.

The leading edge of signal C causes a monostable circuit 25 to generatea trigger pulse F which is delivered on a line 26 to a three μseconddelay circuit 28. The purpose of the delay will be explained later.After that delay, circuit 28 delivers a signal R having a rising edgewhich when applied to a magnetron pulse width monostable circuit 30causes it to generate a firing pulse S whose length is adjustable in amanner which will be described below. The firing pulse S is applied to aswitching circuit 32 which discharges a capacitor 34 which haspreviously been charged to approximately 300 volts (the manner in whichthis charging is accomplished will be described later) through theprimary winding of a pulse transformer 36. This causes a voltage in theregion of 2 kilovolts to be induced in the secondary of pulsetransformer 36, which voltage is applied across the magnetron 2 whichemits a very brief pulse W of energy at 9450 MHz, or thereabouts, fortransmission by the transmitting section 4a of scanner 4. The length ofthe pulse transmitted is variable by the above-mentioned adjustment ofthe length of the firing pulse S applied to the switching circuit 32.

A 30 second delay circuit 37 prevents transmission of the trigger pulseduring 30 seconds after the circuit is first switched on, to ensure thatmagnetron 2 has warmed up before firing pulses are first applied to it.

CRT SCAN

The spot on CRT 8 should start its scan from the centre at the instantwhen the pulse is transmitted from scanner 4. An adjustment whichenables this to be achieved in a manner which will become apparent, isprovided by means of a delay circuit 38 whose delay is adjustablebetween about 1.5 microseconds and 4.5 microseconds. The leading edge of1000 Hz pulse C emerges from delay circuit 38 (waveform G) and isapplied to a range period generator 40 and to a scan coil driver circuit42. This causes the range period generator 40 to start producing anoutput waveform O at a preset frequency and causes the scan coil drivercircuit 42 to start producing a trace period pulse (waveform H), This isa constant voltage pulse (derived from a power supply 110 via connectionx--x) and is applied through an inductive scan speed control circuit 44to the scanning coils 10, which results in a linearly rising current(waveform Q) through the scan coils so as to produce linear deflectionof the CRT beam. Circuit 44 comprises different inductance values whichare selected automatically, through linkage of the circuit with therange switch, such that the total inductance of circuit 44 and thescanning coils 10 results in the gradient of this ramp signal and theconsequent speed of the scanning spot on the tube face being appropriateto the range setting currently in use. From the above, it will beevident that the spot begins to scan from the CRT centre coincident withthe leading edge of waveform G.

Since the timing of the leading edge of waveform G is adjustable byadjustment of delay circuit 38, the commencement of beam scan can bemade coincident with transmission of radar pulse W. Imposition of thefixed 3 μs delay on pulse W enables adjustment of circuit 38 not only toretard, but also to advance, the timing of beam scan initiation relativeto radar pulse transmission, and either may be necessary in a particularinstallation to compensate for unpredictable timing errors caused bycomponent variations, and variations in the length of the cableconnecting the display unit and the scanning unit, from one installationto another.

The duration of the CRT beam scan is controlled as follows. The fixedfrequency pulses O from range period generator 40 are applied to adivider 46. This has a plurality of outputs for different divisionratios and the outputs are selectable by means of a range switch section24b. The division ratios at the different outputs of divider 46 are madesuch that, in conjunction with the predetermined frequency of rangeperiod generator 40, output pulses from the different outputs will bedelivered at times (measured from the commencement of the beam scan)which will coincide with the times at which the scanning spot will reachthe limit of its scan when travelling at the different speeds which areselectable as already explained by means of the range switch and thescanning speed control circuit 44. Consequently, the selected output ofdivider 46 delivers a pulse at the moment when the scanning spot is atits limit and this is applied both to the range period generator 40 toterminate its operation (see waveform O) and to the scan coil drivercircuit 42 to terminate its constant voltage output pulse H, thusremoving current from the scanning coils and allowing the scanning beamto fly back to the centre of the tube.

During the beam scan just described, pulse H is also applied to abright-up circuit 48 which throughout the scan applies to the grid ofCRT 8 a potential which brings the spot to the verge of visibility onthe tube face, so that any significant video signal applied to the tubewill make the spot visible. During the periods between scans of thespot, the bright-up circuit 48 keeps the tube beam suppressed so as toavoid the possibility of burning the centre of the CRT screen. Theoutput of the bright-up circuit 48 is the constant voltage portion ofwaveform N.

RANGE RINGS

The range rings on the CRT display, already referred to, are produced asfollows.

Beam scan pulse H from scan coil driver 42 is applied to a range ringgenerator 50 which throughout the duration of pulse H produces an outputsignal K at constant frequency independently of the other oscillators inthe circuit. This is applied to divider 52 which again has outputscorresponding to diferrent division ratios which are individuallyselectable by means of range switch section 24c. Consequently after eachpredetermined number of pulses K (the number and hence the interveningtime period being settable by range switch section 24c) a pulse fromdivider 52 is applied to a pip generating circuit 54 which generates avery brief pulse (waveform M) which is applied to the grid input of CRT8. The frequency of ring generator 50 and division ratios of divider 52are so arranged that four pip signals from generator 54 will be appliedto the video input of CRT 8 at equal intervals. Consequently, since thetube scan rotates, the display shows four concentric equally spacedrings. The ring positions are the same irrespective of the range beingemployed, since the intervals between pips are selected to match thescanning speed being employed at the different ranges.

SCANNING SYNCHRONISATION

The motor M1 which rotates the scanner 4 is fixedly mounted to anelevated part of the boat and its output shaft indicated by the brokenline 56 carries the scanner 4 and two discs 58 and 60. Disc 58 is formedwith a predetermined number, for example fifty, of transparent portionsor apertures 62 and disc 60 is formed with a transparent portion orcut-out 64. A light emitting and detecting assembly is shownschematically at 66, for directing light towards disc 58 at the radialposition of the apertures 62, and detecting light which passes throughthe apertures. Consequently, the output of the detecting section ofassembly 66 is a train of square pulses as indicated at 68 in FIG. 2, ata frequency proportional to the speed at which the scanner 4 is beingrotated, which typically would be about 1 revolution per second, givinga pulse frequency of 50 pulses per second if disc 58 is provided withfifty apertures 62. Disc 60 is so orientated on shaft 56 that itstransparent portion or cut-out 64 is aligned with a light-emitting anddetector assembly 69 (similar to asssembly 66) when the scanner 4 isdirected towards the head (i.e. forwards exactly along the fore-and-aftaxis) of the boat. The relative orientation of discs 58 and 60 is suchthat the single brief pulses 70 derived from the detection section ofassembly 69 each time the scanner passes the head of the boat aresuperimposed on one of the pulses 68 derived from the synchronisationdisc 58. This combined signal is transmitted on line 72 to the displayunit. In an alternative construction apertures 62 and cut-out 64 may bein a single disc.

At the display unit, the combined signal is applied to a motor drivecircuit 74 which amplifies the received pulses and applies them as adriving signal to motor M2, which is a synchronous motor. Consequently,motor M2, if designed to rotate its output shaft once for every fiftypulses received, will rotate the coils 10 at the same speed as scanner 4and correspondence between the positions of the two will be maintained.It should be noted that because the heading pulses 70 coincide withsynchronisation pulses 68, they do not disturb the synchronisationbetween motors M1 and M2.

HEADING MARK

The combined pulse signal on line 72 is further applied to a thresholdcircuit 76 which passes the heading pulses 70 but not thesynchronisation pulses 68. Pulses 70 are applied from the thresholdcircuit to a heading mark circuit 77. For each pulse, heading markcircuit 77 generates an output signal which lasts as long as two radialsweeps of the CRT spot (at the lowest sweep rate available), and whichis applied to the video input of CRT 8. Thus, a solid radial marker lineappears on the CRT display indicating thereon the boat's heading. Thisheading marker should extend from the screen centre to the exact topcentre of the screen. This implies that when scanner 4 passes the headof the boat, the scan coils 10 in the display unit should be positionedso as to deflect the scanning beam from the CRT screen centre towardsthe exact top centre of the screen. This is achieved by means of aswitch 75 which, when opened, stops circuit 74 from driving motor M2.Consequently, if at the start of operation the scanner and displayangles are not matched, so that the heading marker is not at top centre,switch 75 can be opened to briefly stop motor M2 and angularly shift theheading marker, until it has the correct position on the display, atwhich point the scanner 4 and coils 10 will be in proper angularrelationship.

BEARING MARK

A further aspect of the display is the provision of a bearing mark, thatis to say a radial line on the screen which can be angularly adjusted bya control at the display unit so as to be aligned with any featurevisible on the display. This enables, in conjunction with an angularindex provided around the tube face, the bearing of such a displayfeature relative to the ship's heading (which is made always to coincidewith the top of the screen and the zero position on the angular index)to be read off by an operator.

For this, pulses 70 are applied from the threshold circuit to a bearingmark circuit 78. Bearing mark circuit 78 responds to a heading pulse 70by generating a pulse, the length of which can be adjusted between zeroand the length of time taken for a complete rotation of the scanner 4,by means of a control 80. Further, bearing mark circuit 78 generates,upon termination of the variable-length pulse, an output pulse having alength equal to the time taken for the CRT spot to make two sweeps(again at the lowest available sweep repetition rate). The output pulsefrom bearing mark circuit 78 is applied to gate 82 and holds the gateopen during two (or more) sweeps of the CRT spot. Applied to anotherinput of gate 82 are pulses L, a plurality (in practice typicallysixteen) of these being generated at equal intervals during the spotsweep as can be seen from FIG. 3. Consequently, during at least twosuccessive spot sweeps the spot is brightened e.g. sixteen times persweep so as to display a dashed line on the screen, this being a bearingmark. The angular position of that line about the screen centre isdetermined by adjustment of the control 80 on bearing mark circuit 78which, as will be understood, determines how much time elapses betweenthe CRT scan passing the top centre position and the beam beingbrightened to provide the dashed line. Thus, the desired bearing markcan be positioned on the CRT screen at any angular position to mark thebearing, relative to the ship's heading, of any feature on the display.

The pulses which are gated through gate 82 to provide the dashed bearingmark line are derived from the output signal O of range period generator40, this signal being applied to a divider 84 which once again has aplurality of outputs for different division ratios. An individual outputof divider 84 is selected by means of range switch section 24d. A lowerdivision ratio is selected when the range switch is set to low range(i.e. high spot scanning speed), and vice versa, so that the bearingmark pulses L will be at frequencies proportional to spot scanningspeed, whereby the bearing mark dashes on the screen will be identicalirrespective of which range setting is in use.

RECEIVER

As referred to above, the radar pulse is transmitted from section 4a ofthe scanner at the moment when the CRT beam initiates its radial scan.An echo pulse is received from an object lying in the instantaneousdirection of the scanner 4 after a delay dependent upon the distance ofthe object from the scanner and the echo pulse is received at thereceiving section 4b of the scanner. Its frequency is the same frequencyas the output of magnetron 2. In known manner, the received pulse ismixed in a mixer 86 with the output from a local oscillator 88 togenerate an intermediate frequency which is amplified by intermediatefrequency amplifier 90 and then filtered by filter 92 to provide a videosignal which represents the envelope of the original received echo. Thevideo signal is transmitted on line 94 to the display unit where itpasses through a high pass filter 96 and is applied to the video inputof CRT 8, so as to brighten the scanning spot at a radial distance fromthe screen centre which is proportional to the actual distance from thescanner of the object which produced the echo.

The local oscillator 88 may be of any known kind but it is preferred toemploy an oscillator, such as a Gunn-effect oscillator, whose outputfrequency can be adjusted by adjustment of the level of a d.c. inputsignal. Local oscillator 88 in the described embodiment is of this kind.The d.c. signal for tuning its output frequency is provided by anadjustable tuning circuit 98 at the display unit (FIG. 1) and appliedvia the same line 94 that carries the video signal and via a low-passfilter 100 to a tuning input of local oscillator 88. Since filter 96 isa high-pass filter this d.c. signal does not affect the video input ofCRT 8, and since filter 100 is a low-pass filter, the video signal fromthe output of filter 92 is not fed to the tuning input of localoscillator 88. Thus, by adjusting the d.c. voltage provided by tuningcircuit 98, the frequency of local oscillator 88 can be adjusted so asto provide optimum amplification of the received echo in the receivingsection of the circuitry.

GAIN CONTROL AND SEA CLUTTER CIRCUITRY

A gain control circuit 108 is manually adjustable to generate a constantbut adjustable d.c. gain control voltage which is applied by line 106 toa gain control input of intermediate frequency amplifier 90.

Referring to FIG. 3, waveform I illustrates the form of the video signalwhen echos are being received from rough water. It can be seen that theearliest echoes are of substantial amplitude and would cause the CRTdisplay to have a bright patch around its centre which as alreadymentioned is undesirable.

This can be eliminated by means of the sea clutter circuit 102 (FIG. 1)which receives the trigger pulse F and generates an output signal Jhaving an adjustable period settable by means of a control 104. Thissignal is superimposed on the d.c. gain control voltage from circuit108, on line 106. Consequently, when signal J is low the gain ofamplifier 90 is held at a reduced level so the video signal isattenuated. As signal J gradually rises, the gain of amplifier 90 isincreased until the video signal is amplified in normal manner. Byadjusting control 104 on sea clutter circuit 102, and thus adjusting thetiming of the trailing edge of signal J, the display of echoes receivedfrom rough water around the boat can be attenuated to any desireddistance out from the centre of the display screen.

MAGNETRON CAPACITOR CHARGING

The above explains the events occurring while the CRT spot is sweepingfrom the screen centre to the screen periphery i.e. up to thetermination of ramp signal Q in FIG. 3. It is a feature of the describedembodiment that during this period no power is drawn from the d.c. powersupply circuit 110 for the purpose of recharging the magnetron drivingcapacitor 34, which became partly discharged when the magnetron wasfired immediately after the beginning of the cycle. However, once theCRT scan has been completed a further sequence of events is initiated torecharge capacitor 34 ready for the next radar pulse to be transmitted.This is achieved as follows.

The delay in the initiation of capacitor charging is effected by meansof a stand-off period circuit 112. In response to the leading edge ofsignal R from the 3μs delay circuit 28, the output of stand-off periodcircuit 112 goes low (signal T) for a predetermined period of time. Thatperiod terminates just after the spot scan has been completed at whichtime the output of stand-off period circuit 112 goes high, and theleading edge thus generated triggers a charging period circuit 114which, in response, generates an output signal U having a predeterminedlength. Signal U is applied to a pulse number generator 116, which is anindependent oscillator, and which generates a train of pulses V so longas it is receiving the pulse U from circuit 114. Consequently, thelength of pulse U determines the number of pulses V generated bygenerator 116.

The pulses V are applied to a pulse width monostable circuit 118. Thisgenerates an output pulse for each input pulse received, but the widthof each output pulse can be varied in a manner, and for a purpose, whichwill be described below. The output pulses from pulse width monstable118 are applied to a pulse generating circuit 120 (which will bedescribed in more detail with reference to FIG. 4) which for each inputpulse received applies a pulse of charge to capacitor 34, therebyeventually charging the capacitor to a voltage in the region of 300volts. Charging of capacitor 34 ceases when the predetermined number ofpulses in waveform V have been completed. The circuit is then in acondition ready for transmission of the next radar pulse by energisationof magnetron 2, and simultaneous initiation of the next sweep of the CRTbeam.

On small boats, the standard form of power supply 110 (from whichvarious supply voltages for all those circuit components needing themmay be derived in ways known in themselves) is a 12 volt battery chargedfrom the propulsion motor or from a generator motor. However, theoperating conditions are normally such that the voltage available fromsupply 110 may vary between 11 volts and about 16 volts. In prior radarunits, the circuitry for charging the magnetron energising capacitor hasbeen designed so as to be operable from an input voltage of about 10volts so that there will always be an adequate input voltage for itsoperation despite the fluctuations in the output voltage from the powersupply. However, to avoid excessive loading of the capacitor chargingcircuitry when the supply voltage rises substantially above 10 volts, asis often the case, relatively complex voltage stabiliser circuitry hasbeen employed which in effect dumps power when the output of supply 110is significantly above 10 volts. This power is dissipated in heat sinksand of course represents an undesirable waste of power consumption inthe radar unit. In the embodiment being described, the need for acomplex voltage stabiliser for supplying the capacitor chargingcircuitry, with its attendant cost, and the undesirable waste of powerwhich has been involved, are avoided as follows.

A voltage sensor circuit 122 continually senses the output voltage ofpower supply 110. So long as the sensed voltage remains below a certainthreshold level which will not overload the capacitor chargingcircuitry, voltage sensor 122 produces no output signal on line 124.However, when the sensed voltage rises above this threshold level aproportionate signal is developed on line 124 and applied to thecharging period circuit 114. This is a monostable circuit whose outputpulse (signal U) has a constant length in the absence of a signal online 124 but whose time constant is reduced, with consequent reductionin the length of the output pulse, in proportion to the value of anyinput signal applied on line 124. Consequently, as the supply voltagefrom power supply 110 rises above the threshold value the resultantoutput signal from voltage sensor 122 causes a proportionate reductionin the length of pulse U delivered by charging period circuit 114 topulse number generator 116, and thus a corresponding reduction in thenumber of pulses in signal V applied to the pulse width monostable 118.Consequently the number of pulses (but not their width) in the outputfrom pulse width mono 118 is correspondingly reduced and so is thenumber of pulses of charge delivered by pulse generator 120 to capacitor34. The pulses delivered from pulse generator 120 to capacitor 34 arederived from the power supply 110 and the amount of charge in each pulseis therefore proportional to the instantaneous value of the outputvoltage of supply 110. Consequently, when this voltage rises each pulseapplied to capacitor 34 contains more charge but by means of thecircuitry which has just been described the total number of pulsesapplied to the capacitor 34 to charge it is reduced. Consequently, theamount of charge applied to capacitor 34 is kept substantially constanteven though the output voltage of power supply 110 may rise verysubstantially above the threshold level set at voltage sensor 122. Itcan be appreciated that in this way the circuitry takes only so muchpower as it needs for adequate charging of the capacitor and there is nonecessity for any power to be dumped.

The separation, in time, of the CRT beam scan and the capacitor chargingstep has various advantages. Both these processes require significantcurrent from the power supply 110. By avoiding drawing both currentssimultaneously, line losses in the conductive paths (especially in thosepaths which carry both the currents) are reduced, thus further reducingpower consumption. Also it is the practice to design all circuitry forsatisfactory operation when the power supply voltage is about 10% belowits normal level (e.g. 12 volts). The reduction in peak currentconsumption achieved by non-overlap of the two currents reduces theproportion of this margin which is absorbed by line losses and thereforeless constraint is placed on the design of the circuitry. Also, sincethe high energy transitions involved in capacitor charging do not occurwhile the receiver/CRT scan are active, the internal filtering requiredto prevent the former interfering with the latter can be simplified,saving cost.

FIG. 4

Some aspects of charging and discharging capacitor 34 will now bedescribed in more detail with reference to FIG. 4.

Pulse generator 120 is shown within the broken-line box, supplied withpower from the positive line 126 (see connection Y-Y in FIG. 2) and thecommon line 128 of power supply 110, and receiving its control signalfrom the output of pulse width monostable 118.

Each output pulse from monostable 118 renders conductive atransistorised switching circuit 130 so that a supply voltage pulse isapplied to the primary 132 of a pulse transformer generally indicated at134. This generates an output pulse of higher voltage and of a lengthproportional to the length of the pulse from monostable 118 across thesecondary 136 of the pulse transformer. The polarity of the pulse issuch as to pull charge through a diode 138 onto a capacitor 140 and, viaa diode 142 onto the main capacitor 34. When the voltage acrosssecondary 136 overshoots in the opposite direction further charge isdriven from capacitor 140 through diode 142 onto capacitor 34. Asalready mentioned, the number of pulses thus applied to capacitor 34 isautomatically adjusted so as to ensure that it is kept charged to avoltage of approximately 300 volts.

Charge is stored on capacitor 34 until the firing pulse is delivered bymagnetron pulse width monostable 30. This is applied to the switch shownas block 32 in FIG. 2 which, in the preferred embodiment, is a solidstate switching device having very high input impedance, capable ofswitching pulsed current of up to 16 amps. at 360 volts and with veryfast on and off switching times preferably in the region of 100 ns. Ann-channel enhancement mode VMOS power FET is preferably employed and, inparticular, model VN4000A or VN4001A available from Siliconix Limited ofNewbury, Berkshire, England. The input of the FET is protected by an 18volts Zener diode 144 and the output by six such diodes 146 which limitthe voltage across the output electrodes to 360 volts. When FET 32 isswitched on by pulse S capacitor 34 is discharged through the primary148 of pulse transformer 36, thus generating in its secondary 150 avoltage pulse in the region of 2000 volts which is applied by line 152to the anode 154 of magnetron 2. The resultant oscillation of themagnetron makes available at its output port 155 a radar frequency pulsehaving a power in the vicinity of 1 kw which is applied to thetransmitting section 4a of scanner 4, in known manner, the duration ofthe pulse corresponding to the duration of the output pulse S ofmagnetron pulse width monostable 30.

It should be appreciated that the use of a low powered, low voltagecircuit for generating the firing pulse S, with the pulse-width beingcontrollable by a variable voltage control signal, in conjunction with asolid state switching device with fast switching times and the abilityto switch a high power input provides an economical and reliable way ofenergising a magnetron with controlled width pulses, whether in thecontext of radar equipment or otherwise.

A diode 157 protects switch 32 against the reverse phase overshootvoltage which can be developed by primary 148 in response to thetrailing edge of the pulse induced in secondary 150.

A parallel RC circuit 156, 158, which is not essential for operation,has terminals 160 connected across it, to which an instrument can beconnected to check the magnetron output power.

The above description of the magnetron switching effectively assumesthat the lower end of RC circuit 156, 158 is connected to common line128, so that magnetron anode 154 starts at zero potential as is normal.In fact, a more advantageous arrangement is shown in which the lower endof RC circuit 156, 158 and therefore in effect the anode 154 of themagnetron is connected by a line 166 to a biassing potential which isprovided by a voltage doubling arrangement generally indicated at 168and comprising diodes 170 and 172 and capacitors 174 and 176 connectedas shown. In response to the input pulses applied to capacitor 140, thisproduces a voltage of approximately 600 volts on line 166 so that priorto switching on the magnetron its anode is at approximately 600 voltsrelative to its cathode.

In this way, the voltage needing to be developed across secondarywinding 150 to fire the magnetron is reduced so the turns ratio of pulsetransformer 36 can be reduced. This increases the bandwidth of thetransformer which in turn enables a wider range of pulse widths to beswitched by the transformer than would otherwise be the case.

Also, the amount of power which has to be switched by switch 32 isreduced thus increasing circuit reliability.

The bias applied to the magnetron anode should be below the thresholdlevel at which oscillation could be induced. A lower bias than thatdescribed may be achieved by omitting circuit 168 and connecting thelower end of RC circuit 156, 158 to capacitor 34 to bias the magnetronanode to around 300 volts, but with reduced benefits.

RANGE CHANGING

In a practical embodiment, six different ranges, namely 0.5, 1.0, 2.0,4.0, 8.0 and 16.0 miles are made available. For the first two, the 4000Hz repetition rate is employed by selecting waveform A at range switchsection 24a. For the middle two ranges waveform B (2000 Hz) is selectedand for the two longest ranges waveform C (1000 Hz) is selected.Consequently the operating cycle of radar pulse transmission, CRT beamscan, and capacitor recharge is repeated at one of these three ratesaccording to whether one of the shortest, middle, or longest pairs ofranges is selected.

It has already been mentioned that on a longer range setting a longerradar pulse is desired and on a shorter range setting a shorter radarpulse is desired. Additionally, since the whole operating cycle of thesystem is shorter at short ranges than it is at long ranges, thestand-off period set by circuit 112 needs to be made shorter for shortranges and longer for long ranges, and the length of the capacitorcharging pulses, determined by pulse width monostable 118, needs to begreater when longer radar pulses are to be developed, because a greateramount of power is taken by each radar pulse. The circuits 30, 112 and118 which respectively determine radar pulse length, stand-off periodand charging pulse length are all monostable circuits and it is apreferred feature that the time constants of these circuits bevoltage-controllable. Circuits of this kind are in themselves known, inwhich the circuit time constant is inversely proportional to a d.c.voltage applied to a control terminal of the circuit. For example,Motorola MC145283 monostables may be used with external resistor valuesmade voltage-selectable by diode switching in response to appliedcontrol voltage. Referring to FIGS. 1 and 2, a d.c. control voltagegenerating circuit 162 at the scanning unit is ganged with the rangeswitch and produces an output of 0, 6 or 12 volts (the higher voltagesbeing for the shorter ranges) which is superimposed on the triggersignal F on line 26 for transmission to the scanner unit. At the scannerunit the superimposed d.c. voltage is applied by line 164 and lines 30a,112a and 118a to control terminals on the monostable circuits 30, 112and 118 respectively. (Since a combination of pulse signals (F) and d.c.level signals is present on line 26, suitable isolating and decouplingarrangements are of course provided at circuits 28, 30, 112 and 118 forensuring that they respond only to the desired signals). This enablesthe time constants of these three circuits (which in general will bedifferent from each other) to be selectively set to any of threedifferent values, one for each of the three repetition rates, byoperation of the range switch at the display unit.

The above characteristics of the circuit are set the same for both ofthe ranges in one pair (i.e. longest two, middle two or shortest two).However it is evident that for the two ranges in a pair the scan speedsand scan periods of the CRT beam must differ by a factor of two. Hencesix different division ratios are provided at the output of divider 46,each differing from the next by a factor of two, so as to define sixdifferent values for the length of the output pulse H from scan coildriver 42, one for each range. This sets the required six different scanperiods for the CRT beam and, as already explained, the correspondingdifferent values of inductance set by switching of circuit 44 inresponse to the range switch ensure that the beam will be fullydeflected in each different scan period, thus giving the six scanspeeds.

Further, in order that the range rings and the bearing mark dashes maybe identical irrespective of the CRT scan speed being employed, thefrequencies of the signals for generating them are increased inproportion to scan speed by providing six output frequencies (eachdiffering from the next by a factor of two) from each of dividers 52 and84, the appropriate one being selected at each range due to the gangingof range switch sections 24b, 24c and 24d.

MECHANICAL ARRANGEMENTS

With the exception of the motor M1 and the rotation sensing devices 68and 66, all the components shown in FIG. 2 are contained in a singleassembly with the scanner 4 and rotate with the scanner. The broken line166 indicates diagrammatically the existence and position of five sliprings for transferring power and signals on the five conducting pathswhich link the stationary and rotating parts of the equipment (line 72does not need a slip ring because units 66 and 69 do not rotate). Theuse of slip rings is of course well known for this purpose but in priorradar systems for small boats as many as twelve slip rings have beenrequired.

In the present system, the number of slip rings, and consequently thecost and liability to malfunction, is reduced by superimposing signalshaving different functions on a common conductor, and in particular byputting both the video and local oscillator tuning signals on a singleconductor 94, the gain and sea clutter signals on a single conductor 106and the trigger and transmitted pulse width signals on a singleconductor 26. Further, by also putting both the motor synchronisationand heading indicating signals on a single conductor 72, a total of onlysix conductors are required including the two which carry the positiveand negative power supply potentials to the rotating scanner unit.

These conductors are taken from the scanning unit, which will be locatedat a high point on the boat, to the display unit which will be normallylocated in the hull of the boat, by means of an insulated multicoreconductor which in many cases will be of substantial length. Suchconductors are expensive and therefore the reduction of the number ofcores required enables a substantial saving in the cost of the length ofthe cable.

MODIFICATION OF FIG. 4

Control of the charging of capacitor 34 to provide appropriate energyfor driving the magnetron, in direct response to the level of the supplyvoltage, has already been described with reference to FIGS. 2 and 4.

An alternative form of control, in this case responsive indirectly tosupply voltage, is shown by broken lines in FIG. 4. The mean voltageacross resistor 156 will be proportional to the mean current deliveredto magnetron 2. A control circuit 180 shown in broken lines detects thismean voltage and in response develops a control signal which is appliedon line 182 to the charging period circuit 114, in place of the voltagesensor signal used in the previously described version. Circuit 180 isadapted to cause circuit 114 to increase the charging period (and hencenumber of capacitor charging pulses) in response to a detected fall inmagnetron mean current and to decrease it in response to a rise, wherebyto control capacitor charging such that the magnetron is always fullydriven but not over-driven, irrespective of the supply voltage. Again,this is achieved while charging the capacitor from an unstabilizedsupply voltage without dumping power.

It should be appreciated that the control signal from circuit 180 couldbe used to control capacitor charging by altering the width of thecharging pulses rather than their number.

If the magnetron output pulse repetition frequency and output pulselength are kept in inverse proportion when changing from one range toanother, the desired mean magnetron current will not alter from range torange. However, if this is not the case, control circuit 180 is designedso as to be settable by the range switch so as to control mean magnetroncurrent at different, appropriate, levels depending on the rangesetting.

So far as is known, previous radar equipment for use on small boats hasnot used less than 48 watts power. The various power saving featureswhich have been described above are capable in practice of giving powerconsumption of about half this much, which is a very substantialadvantage since it makes the present system feasible for use on a verylarge number of small boats whose power supply systems would not beadequate to power previously available radar equipment. Further, thisreduction in power consumption may be achieved even using a 90°deflection CRT rather than a 50° deflection CRT as is commonly employedin small boat radar.

I claim:
 1. A circuit for generating high frequency output pulsescomprising a magnetron having input terminals, a pulse transformerhaving primary and secondary windings, the secondary winding beingcoupled to the magnetron input terminals to apply firing pulses thereto,capacitive charge storage means arranged for direct discharge throughthe primary winding of the pulse transformer, a semiconductor switchingdevice having output terminals which are connected in series with saidprimary winding and having a control terminal, and a control circuitadapted to deliver to said control terminal pulses of predeterminedlength, the semiconductor switching device being responsive to a saidpulse at its control terminal to conduct and discharge the chargestorage means through the primary winding of the pulse transformer for aperiod determined by the length of said pulse whereby a firing pulse anda resulting high frequency output pulse are generated which have pulselengths also determined by the length of said pulse.
 2. A circuit asclaimed in claim 1, wherein the semiconductor switching device is apower FET.
 3. A circuit as claimed in claim 2, wherein said power FET isa VMOS power FET.
 4. A circuit as claimed in claim 3, wherein said VMOSpower FET is an n-channel enhancement mode VMOS power FET.
 5. A circuitas claimed in claim 1, wherein said control circuit is adapted todeliver pulses whose lengths are predetermined but variable.
 6. Acircuit as claimed in claim 5, wherein said control circuit is amonostable circuit having voltage-controllable time constant whereby todeliver said variable-length pulses.
 7. A circuit as claimed in claim 1,wherein diode means is connected in parallel with said primary windingand is poled so as to conduct in response to a voltage induced in theprimary winding in response to the trailing edge of a firing pulsegenerated in said secondary winding.
 8. A circuit as claimed in claim 1,wherein voltage limiting means is connected in parallel with theswitching device output terminals.
 9. A circuit as claimed in claim 1,wherein voltage limiting means is connected to the control terminal ofthe switching device.